[oracle] Bug fixes for resource manager metrics (DBMON-3198)

[nenadnoveljic]

What does this PR do?

Fixing bugs for resource manager metrics collection.

Additional Notes

• In RDS plan_name is empty.
• In Oracle 11 plan_name doesn't exist.

Describe how to test/QA your changes

Check that resource manager metrics are being emitted in RDS and Oracle 11. Look for ORA and failed in agent.log (must not appear).

Reviewer's Checklist

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